Recently, a steel sheet for a vehicle requires higher level formability as shape of the vehicle are complicated and integrated. In particular, a bumper reinforcing member and a shock absorber inside a door are required to have high tensile strength and elongation since they closely relate to the safety of passengers of a vehicle in the case of collision. Thus, the bumper reinforcing member and the shock absorber are generally made of a high strength and high ductility steel sheet having a tensile strength of 780 MPa and an elongation 30% or more. As the problem of environmental pollution due to exhaust gas emission is recently rising, researches for light weight vehicles using high strength steel are increasing. However, high strength and high elongation increase the fraction of retained austenite, which has a disadvantage of relatively increasing delayed fracture.
Accordingly, the present invention aims to manufacture a steel sheet for vehicles having high strength and elongation, such as a tensile strength of 980 MPa or more and an elongation of 28% or more, and excellent delayed fracture resistance. A steel sheet containing a great amount of retained austenite for improving both strength and elongation has excellent uniform ductility. This is because retained austenite increases ductility while transforming into martensite when it is deformed. In addition, when localized compression is applied for example in a drawing stage, retained austenite transforming into martensite sharply increases necking resistance. Due to these properties, a cold rolled steel sheet and the like in which a (222) texture is not developed can be subjected to drawing. Therefore, the application of steel sheets containing a great amount of retained austenite having excellent ductility will greatly increase when they can be used as processing products which are subjected to drawing.
Steel sheets containing a great amount of retained austenite are manufactured by two conventional methods.
The first method is an austempering method, which involves adding a great amount of Si and Mn into low carbon steel to form austenite in an annealing stage and then holding a predetermined bainite temperature in a cooling stage to increase both strength and ductility. The retained austenite formed as above is caused to transform into martensite during plastic deformation, thereby increasing strength as well as ductility by alleviating stress concentration. This is referred to as Transformation Induced Plasticity (TRIP) and the resultant steel is used as high strength steel. A first method proposed by the present invention is to manufacture a steel sheet having a composition of the present invention by using the above described continuous annealing method.
The second method is an reverse transformation method, which reverse transforms martensite into austenite by re-annealing Mn low carbon steel at a predetermined temperature after hot rolling. In this method, a mixed texture of martensite and bainite, obtained after the hot rolling, is subjected to cold rolling and then batch annealing to form austenite in lath boundaries of the entire texture, followed by cooling down and retaining at room temperature.
However, as is known up to the present, the steel sheet containing a great amount of retained austenite, manufactured according to the above method, has a problem of delayed fracture in which cracks occur as time passes after drawing (CAMP-ISIJ Vol. 5 (1992), 1841). The delayed fracture frequently occurs in high strength steel, such as a high tensile bolt in 1.2 GPa level, or austenite-based stainless steel. The delayed fracture is generally in the form of cracks, which are caused by the diffusion of hydrogen atoms or molecules under high residual stress (Material Science and Technology, Vol. 20 (2004), 940).
A steel sheet containing a great amount of retained austenite is subjected to delayed fracture since internal stress occurs in boundaries, caused by cubical expansion induced by transformation of retained austenite into martensite by a drawing stage, and concentration increases due to intrusion of hydrogen (Material Science and Engineering A 438-440 (2006), 262-266). In particular, since hydrogen diffusion rate is high and hydrogen solubility is low in a martensite structure, intrusion hydrogen easily collects in boundaries between martensite and retained austenite.
Japanese Laid-Open Patent Application No. 1993-070886 discloses a composition consisting of 0.05 to 0.3% C, 2.0% or less Si, 0.5 to 4.0% Mn, 0.1% or less P, 0.1% S, 0 to 5.0% Ni, 0.1 to 2.0% Al, and 0.01% or less N, where Si (%)+Al (%)≧0.5, and Mn (%)+⅓Ni (%)≧1.0, and also has a structure containing 5% or more retained austenite by volume. A steel slab having the above composition is hot-rolled, coiled at a temperature range from 300 to 720° C., and cold-rolled at a reduction rate from 30 to 80%. The resulting steel sheet is subjected, in the course of a subsequent continuous annealing stage, to heating up to a temperature in the region between Ac1 transformation point and Ac3 transformation point, and then subjected, in the course of cooling, to holding at a temperature range from 550 to 350° C. for 30 secs or more or to slow cooling at a cooling rate of 400° C./min or less. This technology belongs to the class of the continuous annealing, corresponding to the first method of the present invention. However, this technology is different from the present invention since added elements such as Mn, Ti, B and Sb are different and its mechanical properties are greatly less than those of the present invention.
Japanese Laid-Open Patent Application No. 2003-138345 discloses a composition consisting of, by mass, 0.06 to 0.20% C, 2.0% or less Si, and 3.0 to 7.0% Mn, and the balance Fe, in which the volume ratio of retained austenite is 10 to below 20%, and the area ratio of tempered martensite and tempered bainite is 30% or more. A steel ingot having the above composition is manufactured by hot rolling or cold rolling at a reduction rate of 20% or less, followed by tempering heat treatment of holding at 700° C. to (A1 point −50)° C. for 20 sec or less. The resultant steel has a tensile strength of 800 MPa and an elongation of about 30%. Compared with the present invention, this technology has a problem of delayed fracture due to the lack of Al and is different from the present invention with respect to hot finish rolling temperature, cold reduction rate and annealing holding time, and its mechanical properties are greatly less than those requested.
Japanese Laid-Open Patent Application No. Hei 07-138345 discloses a high strength steel sheet consisting of 2 to 6% Mn and 20% or more retained austenite. This steel sheet has a composition consisting of 0.1 to 0.4% C, 0.5% or less Si, 2.0 to 6.0% Mn, 0.005 to 0.1% Al. This steel sheet is produced by subjecting a hot rolled sheet or a cold rolled sheet, which is preliminarily heat-treated at a temperature range from 800 to 950° C. and then air-cooled or cooled at a cooling velocity equal to or higher than air cooling velocity, or a hot rolled sheet, prepared by hot rolling and coiling at a temperature range from 200 to 500° C., or a cold rolled sheet, prepared by cold-rolling this hot rolled sheet, to first-stage annealing at a temperature range from 650 to 750° C. for 1 minute or more, to cooling down to a temperature 500° C. or less, and successively to second-stage annealing at a temperature range from 650 to 750° C. for 1 minute or more. This technology is different from the present invention in that 20% or more retained austenite causes delayed fracture owing to transformation into martensite during drawing and Al for enhancing delayed fracture resistance is not added to the composition. Also with respect to annealing heat treatment, this technology performing the two annealing stages is different from the present invention performing one annealing stage.
While the above described technologies were developed in view of increasing the content of retained austenite in order to increase both strength and ductility, there have been no solutions to the probability of delayed fracture that increases with the amount of retained austenite. Therefore, there are required an alloy composition, which can increase the content of retained austenite as well as improve delayed fracture resistance in order to increase both strength and ductility, and a manufacturing method thereof.